[ServiceWorker] Implement Headers class.

I will replace HeaderMap class with this in another change.

http://fetch.spec.whatwg.org/#headers-class

BUG=373120

Review URL: https://codereview.chromium.org/358573002

git-svn-id: svn://svn.chromium.org/blink/trunk@177088 bbb929c8-8fbe-4397-9dbb-9b2b20218538